In Release 8 (Rel. 8) of 3GPP-LTE (3rd Generation Partnership Project Radio Access Network Long Term Evolution) (hereinafter may be simply referred to as “LTE”), OFDMA (orthogonal frequency division multiple access) is employed as a downlink communication method, and SC-FDMA (single carrier frequency division multiple access) is employed as an uplink communication method.
In the Rel. 8 downlink, in order to demodulate a data signal (for example, a signal transmitted via a PDSCH), a CRS (cell specific reference signal) is used. In other words, the Rel. 8 downlink supports “CRS-used data transmission.” “CRS-used data transmission” refers to a transmission method in which a data signal is transmitted together with a CRS in a subframe with the CRS mapped therein, and during data reception, a terminal estimates a propagation channel according to the CRS and demodulates the data. A CRS is a reference signal to be transmitted over a full band in all subframes and is common within a given cell. Also, CRSs are mapped to time and frequency resources that depend on a cell ID, and for the CRSs, antenna ports 0 to 3 are used according to the number of transmission antennae. Also, CRSs are transmitted so as to cover all areas in a given cell. CRSs are also used for quality measurement, and results of the quality measurement are used for link adaptation or scheduling.
Meanwhile, Rel. 10, which is LTE-Advanced, supports “DMRS (demodulation reference signal)-used data transmission” in order to apply MIMO (multi-input multi-output) to downlink. “DMRS-used data transmission” refers to a transmission method in which a data signal is transmitted together with a DMRS in a subframe with a DMRS mapped therein, and during data reception, a terminal estimates a propagation channel according to the DMRS to demodulate the data. DMRS may be called “UE specific reference signal.” Also, while CRSs are transmitted to an entire cell, DMRSs are transmitted to a terminal to which a data resource for mapping a downlink data signal has been allocated, and transmitted only in a resource block (that is, a frequency resource) to which the data for the terminal has been allocated. When a data signal is transmitted to a predetermined terminal, a beam is formed by precoding, enabling data communication using the beam. Data communication using such beam provides a high throughput (see, for example, NPLs 1, 2, 3 and 4). Also, DMRS-used data transmission can be used for a terminal for which transmission mode 9has been set. Also, antenna ports 7 to 14 are used according to the number of transmission antennae. Also, in Rel. 10, which is LTE-Advanced, CSI-RSs are used for quality measurement, and results of the quality measurement are used for link adaptation or scheduling.
Also, in Rel. 10, which is LTE-Advanced, a terminal for which transmission mode 9 has been set can also transmit a data signal in an “MBSFN (multi-broadcast single frequency network) subframe.”
Meanwhile, in Rel. 8, an “MBSFN subframe” is used for transmitting MBMS data (multicast or broadcast data) from a plurality of base stations to an SFN (single frequency network). Thus, resources in which a PDCCH signal and a CRS are mapped are limited to first two OFDM symbols in a subframe. Then, only MBMS data can be mapped in a third OFDM symbol from the head of the subframe and OFDM symbols subsequent to the third OFDM symbol. In other words, an MBSFN subframe contains no CRS in a third OFDM symbol from the head of the subframe or OFDM symbols subsequent to the third OFDM symbol (that is, data transmission region).
On the other hand, in Rel. 10, which is LTE-Advanced, DMRS-used data transmission (unicast data transmission) can be performed also in MBSFN subframes. As described above, an MBSFN subframe contains no CRS in the third OFDM symbol from the head of a subframe or OFDM symbols subsequent to the third OFDM symbol (that is, data transmission region), and thus, in Rel. 10, which is LTE-Advanced, more time and frequency resources can be used for PDSCH.
Also, for Rel. 11 (release following Rel. 10), which is LTE-Advanced, CoMP transmission, which provides coordinated transmission from a plurality of nodes, is being studied. Also, in the case where the CoMP transmission is used in a heterogeneous network environment, an operation using a cell ID that is the same as that of an HPN for a plurality of LPNs in a macro cell is being discussed (see, for example, NPL 6). In such operation, a common CRS is transmitted from an HPN and LPNs using a same cell ID. The term “heterogeneous network environment” refers to a network environment including a macro base station (HPN (high power node)) and pico base stations (LPN (low power nodes)).
Furthermore, an extension carrier (non-backward compatible carrier) for downlink is being studied for Rel. 11, which is LTE-Advanced. The extension carrier supports only DMRS, and no CRS is transmitted for overhead reduction (see, for example, NPL 7). As described above, the extension carrier enables highly-efficient transmission by the operation that supports DMRS-used data transmission only.
Also, in LTE and LTE-Advanced, a terminal transmits an RACH (random access channel) to a base station when initial access is made, uplink data is generated during connection, or a handover is performed. Consequently, an attempt to establish connection from the terminal to the base station or to establish re-synchronization therebetween is made. A series of operations for the connection from the terminal to the base station or the establishment of re-synchronization therebetween is called “random access procedure.” “Random access procedure” includes the four steps indicated in FIG. 1 (see, for example, NPL 5).
Step 1 (transmission of message 1): A terminal randomly selects an RACH preamble resource to be actually used from a group of RACH preamble resource candidates (prescribed by combinations of time resources, frequency resources and sequence resources). Then, the terminal transmits an RACH preamble using the selected RACH preamble resource. Here, the selectable RACH preamble resource candidates are different depending on whether a propagation loss (path loss) between a base station and the terminal is not less than a predetermined threshold or is not greater than the predetermined threshold. The selectable RACH preamble resource candidates are also different depending on whether the data size is not less than a predetermined threshold or is not greater than the predetermined threshold. Also, an RACH preamble may be called “message 1.”
Step 2 (transmission of message 2): when a base station detects the RACH preamble, the base station transmits an RACH response (or a random access response). At this point of time, the base station cannot identify the terminal that has transmitted the RACH preamble. Thus, the RACH response is transmitted over the entire cell covered by the base station. A data resource in which the RACH response is mapped (that is, a PDSCH resource) is indicated by the base station to the terminal via a PDCCH. Also, the RACH response contains information relating to a resource to be used by the terminal in uplink or information relating to uplink transmission timing for the terminal. Here, if the terminal that has transmitted the RACH preamble receives no RACH response within a predetermined period of time from the transmission of the RACH preamble (that is, a retransmission determination period), the terminal performs RACH preamble resource selection and RACH preamble transmission (RACH retransmission) again.
Step 3 (transmission of message 3): the terminal transmits data such as an RRC connection request or a scheduling request using the uplink resource specified by the base station via the RACH response.
Step 4 (transmission of message 4): The base station transmits a message containing a UE-ID (for example, C-RNTI or temporary C-RNTI) assigned to the terminal, to the terminal to confirm that there is no contention between a plurality of terminals (contention resolution).